The invention relates to a filter unit for a static power converter, comprising a plurality of electrochemical capacitors formed by coils with parallel axes, positioned on a mechanical support, and cooperating at the opposite end with an electrical connecting device, said capacitors being divided into different groups connected in parallel to the power supply terminals, each group comprising at least two series-mounted capacitors, whose mid-point voltage is fixed by a voltage divider with balancing resistors R.
FIG. 1 shows the wiring diagram of a filter cell 10 of the prior art with two electrochemical capacitors C1 and C2 parallel connected by a connecting device. Z1 and Z2 represent the partial impedances of the bars connecting the connecting device in parallel. When the cell 10 is supplied between the points A and D, the distribution of the currents depends only on the impedances of the two capacitors C1 and C2, given that each capacitor is connected to the opposing polarities of the power supply by a connecting impedance having a value of 2 Z1+Z2. The equivalent diagram between the points A and D is represented in FIG. 2. A filter cell 10 of this kind can be suitable for a DC power supply whose service voltage is lower than 300 volts.
For DC power supplies having operating voltages of higher values, for example 450 volts DC, each capacitor C1, C2 in FIG. 1 has to be replaced by a group of two series-mounted capacitors, because electrochemical capacitors available in the trade generally have a rated voltage of 350 V DC. Distribution of the DC voltages with groups of series-mounted capacitors is achieved in a conventional manner by means of the block diagram of the filter cell 20 illustrated in FIG. 3. The cell 20 comprises four capacitors 12, 14, 16, 18 arranged in two groups of two, connected in parallel to the power supply terminals. Z1 and Z2 represent the partial impedances of the connecting bars for parallel connection of the two groups. The mid-point 22, 24 of each group is connected to a voltage divider 26, with two resistors R having the same value. Each capacitor 12 to 18 is connected to the respective mid-point 22, 24 by a connecting impedance Z3/2. When the direct current flowing through the resistors R is great in relation to the leakage current of the capacitors 12 to 18, each resistive divider 26 imposes the voltage at the mid-point 22, 24 of each group in such a way as to balance the DC voltage at the terminals of the capacitors 12, 14, 16, 18.
A filter bank DC voltage distribution principle of this kind is used in the ALPES 4000 static converter marketed by the applicant. The twin-resistor voltage divider is arranged outside the enclosure housing the elementary capacitors, and is common for all the groups of two series-mounted capacitors. The mid-point of the resistive divider is connected to the respective mid-point of each group of capacitors by an individual electrical connection with an additional resistor. Depending on the power of the converter, each filter cell enclosure comprises a different number of groups of series-mounted capacitors, and of individual wired connections with the common voltage divider. The dimensions of the enclosure are specific for each power, which gives rise to a storage problem.
The same is true of the connecting device with bars connecting the elementary capacitors in parallel. Each filter bank power has corresponding to it a given type of prefabricated connecting device designed to connect a predetermined number of capacitors.
The object of the invention is to avoid the above-mentioned drawbacks and to perfect standardization of the filter banks of a range of power converters.